Multiple user controls share collection dependency property

I have implemented my own usercontrol based on listboxes. It has a dependency property with type of a collection. It works fine when I have only one instance of the usercontrol in a window, but if I have multiple instances I get problem that they share the collection dependency property. Below is a sample illustrating this.

My user control called SimpleList:

<UserControl x:Class="ItemsTest.SimpleList"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Name="_simpleList">
    <StackPanel>
        <TextBlock Text="{Binding Path=Title, ElementName=_simpleList}" />
        <ListBox 
            ItemsSource="{Binding Path=Numbers, ElementName=_simpleList}">
            <ListBox.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Horizontal" />
                </ItemsPanelTemplate>
            </ListBox.ItemsPanel>
        </ListBox>
    </StackPanel>    
</UserControl>

Code behind:

using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;

namespace ItemsTest
{
    public partial class SimpleList : UserControl
    {
        public SimpleList()
        {
            InitializeComponent();
        }

        public string Title
        {
            get { return (string)GetValue(TitleProperty); }
            set { SetValue(TitleProperty, value); }
        }

        public static readonly DependencyProperty TitleProperty =
            DependencyProperty.Register("Title", typeof(string), typeof(SimpleList), new UIPropertyMetadata(""));


        public List<int> Numbers 
        {
            get { return (List<int> )GetValue(NumbersProperty); }
            set { SetValue(NumbersProperty, value); }
        }

        public static readonly DependencyProperty NumbersProperty =